U.S. patent application number 10/857480 was filed with the patent office on 2004-12-02 for optical fiber laser and laser light emitting method.
This patent application is currently assigned to FUJIKURA LTD.. Invention is credited to Okada, Yasuyuki, Sakai, Tetsuya, Segi, Takeshi.
Application Number | 20040240488 10/857480 |
Document ID | / |
Family ID | 33303703 |
Filed Date | 2004-12-02 |
United States Patent
Application |
20040240488 |
Kind Code |
A1 |
Okada, Yasuyuki ; et
al. |
December 2, 2004 |
Optical fiber laser and laser light emitting method
Abstract
An optical fiber laser includes a resonator comprising an
erbium-doped glass fiber serving as a gain medium and a pumping
light source that launches pumping light into the erbium-doped
glass fiber. The pumping light source emits pumping light of a
wavelength of 980 nm or longer, and the optical fiber laser emits
laser light in a wavelength band of 2.8 .mu.m (2.7 .mu.m to 3.0
.mu.m).
Inventors: |
Okada, Yasuyuki;
(Sakura-shi, JP) ; Segi, Takeshi; (Sakura-shi,
JP) ; Sakai, Tetsuya; (Sakura-shi, JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 Pennsylvania Avenue, N.W.
Washington
DC
20037-3213
US
|
Assignee: |
FUJIKURA LTD.
|
Family ID: |
33303703 |
Appl. No.: |
10/857480 |
Filed: |
June 1, 2004 |
Current U.S.
Class: |
372/6 ;
372/70 |
Current CPC
Class: |
H01S 3/094092 20130101;
H01S 3/1608 20130101; H01S 3/067 20130101; H01S 3/094003 20130101;
H01S 3/094096 20130101 |
Class at
Publication: |
372/006 ;
372/070 |
International
Class: |
H01S 003/30 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 2, 2003 |
JP |
P2003-156988 |
Jun 10, 2003 |
JP |
P2003-165075 |
Nov 12, 2003 |
JP |
P2003-382617 |
Claims
What is claimed is:
1. An optical fiber laser comprising: a resonator comprising an
erbium-doped glass fiber serving as a gain medium; and a pumping
light source that launches pumping light into said erbium-doped
glass fiber, wherein said pumping light source emits pumping light
of a wavelength of 980 nm or longer, and said optical fiber laser
emits laser light in a wavelength band of 2.8 .mu.m.
2. The optical fiber laser according to claim 1, wherein the
wavelength of said pumping light is between 985 nm and 1000 nm.
3. An optical fiber laser comprising: a resonator comprising an
erbium-doped glass fiber serving as a gain medium; and a plurality
of pumping light sources that launch pumping light into said
erbium-doped glass fiber, wherein said plurality of pumping light
sources include a first pumping light source that emits first
pumping light that causes ground state absorption and a second
pumping light source that emits second pumping light that causes
excited state absorption from a .sup.4I.sub.13/2 level.
4. The optical fiber laser according to claim 3, wherein said
optical fiber laser emits laser light in a wavelength band of 2.8
.mu.m due to transition of erbium ions from a .sup.4I.sub.11/2
level down to the .sup.4I.sub.13/2 level.
5. The optical fiber laser according to claim 4, wherein said
second pumping light does not cause excited state absorption from
the .sup.4I.sub.11/2 level.
6. The optical fiber laser according to claim 3, wherein the
wavelength of said first pumping light is between 960 nm and 1020
nm.
7. The optical fiber laser according to claim 3, wherein the
wavelength of said second pumping light is between 780 nm and 792
nm.
8. The optical fiber laser according to claim 7, wherein the
wavelength of said first pumping light is between 960 nm and 1020
nm.
9. A laser light emitting method for emitting laser light in a
wavelength band of 2.8 .mu.m by launching pumping light into a
resonator comprising an erbium-doped glass fiber serving as a gain
medium, wherein said pumping light is light of a wavelength of 980
nm or longer.
10. The laser light emitting method according to claim 9, wherein
the wavelength of said pumping light is between 985 nm and 1000
nm.
11. A laser light emitting method for emitting laser light by
launching a plurality of rays of pumping light into a resonator
comprising an erbium-doped glass fiber serving as a gain medium,
wherein, as said plurality of rays of pumping light, first pumping
light that causes ground state absorption and second pumping light
that causes excited state absorption from a .sup.4I.sub.13/2 level
are concurrently used.
12. The laser light emitting method according to claim 11, wherein
laser light in a wavelength band of 2.8 .mu.m is emitted by said
pumping light causing transition of erbium ions from a
.sup.4I.sub.11/2 level down to the .sup.4I.sub.13/2 level.
13. The laser light emitting method according to claim 11, wherein,
as said pumping light, first pumping light is in a wavelength range
of 960 nm to 1020 nm and second pumping light is in a wavelength
range of 780 nm to 792 nm, and the first pumping light and the
second pumping light are concurrently used.
14. A laser light emitting method comprising: launching a first
pumping light having a first wavelength into a resonator having an
erbium-doped glass fiber serving as a gain medium, exciting erbium
ions of the erbium-doped glass fiber from a ground state to a first
energy level; launching a second pumping light having a second
wavelength into the resonator, exciting erbium ions from a second
energy level to a third energy level, wherein said second energy
level is between the ground state and the first energy level, and
said third energy level is higher than said first energy level; and
emitting laser light from the resonator resulting from excited
erbium ions decaying from the first energy level to the second
energy level, wherein said launching of the second pumping light
having the second wavelength does not excite erbium ions from the
first energy level to a higher energy level.
15. The laser light emitting method according to claim 14, wherein
the launching of the first pumping light, the launching of the
second pumping ligth, and the emitting of the laser light are
concurrent.
16. The laser light emitting method according to claim 14, wherein
the first wavelength and the second wavelength are different.
17. The laser light emitting method according to claim 15, wherein
the first wavelength and the second wavelength are different.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an optical fiber laser
incorporating an erbium doped fiber as a gain medium. In
particular, it relates to an optical laser that emits laser light
in the wavelength band of 2.8 .mu.m (2.7 .mu.m to 3.0 .mu.m) and to
a method of emitting the laser light.
[0003] This application claims priority on Japanese Patent
Application No. 2003-156988 filed on Jun. 2, 2003, Japanese Patent
Application No. 2003-165075 filed on Jun. 10, 2003 and Japanese
Patent Application No. 2003-382617 filed on Nov. 12, 2003, of which
are incorporated herein.
[0004] 2. Description of the Related Art
[0005] Lasers operating in the wavelength range of 2.8 .mu.m to 2.9
.mu.m, which is the water absorption band, are used in the medical
field and the like. As such a laser operating in the wavelength
range of 2.8 .mu.m to 29 .mu.m, there has been proposed an optical
fiber laser incorporating an erbium doped fiber (also referred to
as EDF, hereinafter) as a gain medium (for example, see Applied
Physics B by M. Pollnau et al., 1998, Vol. 67, pp. 23-28). In
general, the optical fiber emits laser light in the wavelength band
of 2.8 .mu.m (2.7 .mu.m to 3.0 .mu.m) using pumping light in a
wavelength of shorter than 980 nm.
[0006] In order to increase the power of the laser light of the
optical fiber laser described above, the EDF serving as the gain
medium must be longer. However, the longer EDF leads to a longer
resonator (or a longer resonator length), resulting in a reduced
peak power of the laser light and a larger temporal half-width of
the laser light.
[0007] Thus, there is a need to enhance the emission efficiency of
laser light to achieve a high laser power with a relatively short
resonator length.
[0008] FIG. 7 is an energy level diagram for erbium ions of an
erbium doped fiber.
[0009] When pumping light in a wavelength band of 980 nm is
launched into the EDF, ions in the ground state (.sup.4I.sub.15/2)
are excited to the upper level (.sup.4I.sub.11/2) due to ground
state absorption (also referred to as GSA, hereinafter). Then, when
the ions decay to the lower level (.sup.4I.sub.13/2) from the upper
level (.sup.4I.sub.11/2), the laser light in a wavelength band of
2.8 .mu.m (2.7 .mu.m to 3.0 .mu.m) is emitted.
[0010] If excited state absorption (also referred to as ESA,
hereinafter) occurs in the upper level (.sup.4I.sub.11/2), the ions
are excited to a still higher energy level (.sup.4F.sub.7/2), and
the emission efficiency of the laser light in a wavelength band of
2.8 .mu.m (27 .mu.m to 3.0 .mu.m) is reduced.
[0011] Recently, there has been a report on absorption spectra of
GSA and ESA in a wavelength band of 980 nm for the EDF (for
example, see the "Proceedings of the Meeting: Fiber laser sources
and amplifiers" by Richard Quimby, The International Society for
Optical Engineering, 1991, vol. 1581, pp. 72-79).
[0012] If the ESA is reduced, a higher emission efficiency can be
achieved, and the laser power of the optical fiber can be raised
without elongation of the optical fiber. However, research
concerning the excitation condition has been insufficient, and the
optimum wavelength of the pumping light has not been found yet.
[0013] FIG. 8 is a graph showing a relationship between the peak
wavelength of laser light emitted from a conventional optical fiber
laser and the power of the pumping light.
[0014] The conventional optical fiber laser uses one pumping light
in the wavelength of shorter than 980 nm. As can be seen from the
drawing, as the power of the pumping light increases, the peak
wavelength of the emitted laser light is shifted toward longer
wavelengths. Thus, the conventional optical fiber laser has a
problem that the peak wavelength of the laser light varies with the
power of the pumping light when adjusting the power of the pumping
light to achieve a desired value of the laser light power. For
example, in the case in which the optical fiber laser is used for
medical purposes, if the peak wavelength of the laser light varies,
there arises a problem of a reduced capability of cutting or
ablating of living tissues.
[0015] The mechanism of how this problem occurs will be described
below.
[0016] In general, each energy level at the time of laser emission
is often represented as one line as shown in FIG. 9. However,
actually, each level splits into several sub-levels at narrow
intervals as shown in FIG. 10. These sub-levels are referred to as
Stark levels. The wavelength of the emitted laser light (emitted
light wavelength) depends on the Stark levels between which the ion
transition causing the light emission occurs.
[0017] If the intensity (power) of the pumping light is increased,
ions at the ground state level (.sup.4I.sub.15/2) shown in FIG. 9
are excited. Therefore the number of ions occupying the upper level
of erbium (.sup.4I.sub.11/2) and the lower level of erbium
(.sup.4I.sub.13/2) increase.
[0018] Since lower Stark levels are occupied by ions earlier than
higher ones in each energy level, the lower Stark levels in the
lower level (.sup.4I.sub.13/2) are also occupied by ions earlier
than the higher ones.
[0019] As the intensity (power) of the pumping light increases, the
lower Stark levels of the lower level (.sup.4I.sub.13/2) are
occupied by ions and become incapable of contributing to light
emission, and thus, the higher Stark levels, which are occupied by
fewer ions, participate in light emission. As a result, the
interval between the lower level (.sup.4I.sub.13/2) and the upper
level (.sup.4I.sub.11/2) is reduced, and thus, the wavelength of
the laser light (emitted light wavelength) is disadvantageously
shifted toward longer wavelengths.
SUMMARY OF THE INVENTION
[0020] The present invention was made in view of the circumstances
described above.
[0021] A first object of the present invention is to provide an
optical fiber laser that has a high light emission efficiency and
can achieve a high laser power.
[0022] A second object of the present invention is to provide an
optical fiber laser that can stably emit laser light of a constant
wavelength, preventing the peak wavelength of the emitted laser
light from being shifted even if the intensity (power) of the
pumping light varies.
[0023] An optical fiber laser according to the first aspect of the
present invention includes: a resonator comprising an erbium doped
glass fiber serving as a gain medium; and a pumping light source
that launches pumping light into the erbium-doped glass fiber, in
which the pumping light source emits pumping light of a wavelength
of 980 nm or longer, and the optical fiber laser emits laser light
in a wavelength band of 2.8 .mu.m (2.7 .mu.m to 3.0 .mu.m).
[0024] According to this aspect of the present invention, since the
wavelength of the pumping light is 980 nm or longer, the laser
light in the wavelength band of 2.8 .mu.m (2.1 .mu.m to 3.0 .mu.m)
of a high power can be emitted with a high efficiency.
[0025] The wavelength of the pumping light may be between 985 nm
and 1000 nm. In this case, the laser light of a high power can be
emitted with a high efficiency compared with the case in which
conventional pumping light of a wavelength of shorter than 980 nm
is used.
[0026] An optical fiber laser according to the second aspect of the
present invention includes: a resonator comprising an erbium doped
glass fiber serving as a gain medium; and a plurality of pumping
light sources that launch pumping light into the erbium-doped glass
fiber, in which the plurality of pumping light sources include a
first pumping light source that emits first pumping light that
causes ground state absorption and a second pumping light source
that emits second pumping light that causes excited state
absorption from a .sup.4I.sub.13/2 level.
[0027] According to this aspect of the present invention, even if
the intensity (power) of the pumping light varies, the laser light
of a constant wavelength can be stably emitted with a high
efficiency while preventing the peak wavelength of the laser light
from being shifted toward longer wavelengths.
[0028] The optical fiber laser may emit laser light in a wavelength
band of 2.8 .mu.m (2.7 .mu.m to 3.0 .mu.m) due to transition of
erbium ions from a .sup.4I.sub.11/2 level down to the
.sup.4I.sub.13/2 level.
[0029] The second pumping light may not cause excited state
absorption from the .sup.4I.sub.11/2 level.
[0030] The wavelength of the first pumping light may be between 960
nm and 1020 nm. In this case, the laser light in a wavelength range
of 2.8 .mu.m to 2.0 .mu.m of a high power can be emitted with a
high efficiency.
[0031] The wavelength of the second pumping light may be between
780 nm and 792 nm. In this case, it is possible to cause ESA by the
erbium ions at the lower level (.sup.4I.sub.13/2) without causing
ESA by the erbium ions at the upper level .sup.4I.sub.11/2). Thus,
the laser light of a constant wavelength can be emitted further
stably while preventing the peak wavelength thereof from being
shifted toward longer wavelengths.
[0032] A laser light emitting method according to the first aspect
of the present invention is a method for emitting laser light in a
wavelength band of 2.8 .mu.m (27 .mu.m to 3.0 .mu.m) by launching
pumping light into a resonator comprising an erbium doped glass
fiber serving as a gain medium, in which the pumping light is light
of a wavelength of 980 nm or longer.
[0033] According to this aspect of the present invention, the laser
light in the wavelength band of 2.8 .mu.m (2.7 .mu.m to 30 .mu.m)
of a high power can be emitted with a high efficiency.
[0034] The wavelength of the pumping light may be between 985 nm
and 1000 nm. In this case, the laser light of a high power can be
emitted with a high efficiency compared with the case where
conventional pumping light of a wavelength of shorter than 980 nm
is used.
[0035] A laser light emitting method according to the second aspect
of the present invention is a method for emitting laser light by
launching a plurality of rays of pumping light into a resonator
comprising an erbium-doped glass fiber serving as a gain medium, in
which, as the plurality of rays of pumping light, first pumping
light that causes ground state absorption and second pumping light
that causes excited state absorption from a .sup.4I.sub.13/2 level
are concurrently used.
[0036] According to this aspect of the present invention, even if
the intensity (power) of the pumping light varies, the laser light
of a constant wavelength can be stably emitted with a high
efficiency while preventing the peak wavelength of the laser light
from being shifted toward longer wavelengths.
[0037] Laser light in a wavelength band of 2.8 .mu.m (2.7 .mu.m to
3.0 .mu.m) may be emitted by the pumping light causing transition
of erbium ions from a .sup.4I.sub.11/2 level down to the
.sup.4I.sub.13/2 level.
[0038] As the pumping light, first pumping light may be in a
wavelength range of 960 nm to 1020 nm and second pumping light may
be in a wavelength range of 780 nm to 792 nm, and the first pumping
light and the second pumping light may be concurrently used In this
case, the laser light of a constant wavelength between 2.8 .mu.m
and 2.9 .mu.m of a high power can be emitted stably with a high
efficiency.
[0039] A laser light emitting method according to the third aspect
of the present invention is a method for emitting laser light by
launching a first pumping light having a first wavelength into a
resonator having an erbium-doped glass fiber serving as a gain
medium, exciting erbium ions of the erbium-doped glass fiber from a
ground state to a first energy level, and launching a second
pumping light having a second wavelength into the resonator,
exciting erbium ions from a second energy level to a third energy
level, in which the second energy level is between the ground state
and the first energy level, and the third energy level is higher
than the first energy level, and emitting laser light from the
resonator resulting from excited erbium ions decaying from the
first energy level to the second energy level, in which the
launching of the second pumping light having the second wavelength
does not excite erbium ions from the first energy level to a higher
energy level.
[0040] The launching of the first pumping light, the launching of
the second pumping ligth, and the emitting of the laser light may
be concurrent.
[0041] The first wavelength and the second wavelength may be
different.
BRIEF DESCRIPTION OF THE DRAWINGS
[0042] FIG. 1 is a schematic diagram showing an optical fiber laser
of an embodiment according to an aspect of the present
invention;
[0043] FIG. 2 is a graph showing a relationship between the power
of laser light in the wavelength band of 2.8 .mu.m (2.7 .mu.m to
3.0 .mu.m) emitted from an exemplary optical fiber laser based on
FIG. 1 and the wavelength of the pumping light;
[0044] FIG. 3 is a schematic diagram showing an optical fiber laser
of an embodiment according to another aspect of the present
invention;
[0045] FIG. 4 is a graph showing absorption spectra of the GSA and
ESA by erbium ions;
[0046] FIG. 5 is a graph showing a relationship between the peak
wavelength of laser light in the wavelength band of 2.8 .mu.m (2.7
.mu.m to 3.0 .mu.m) emitted from an exemplary optical fiber laser
based on FIG. 3 and the power of the pumping light;
[0047] FIG. 6 is a graph showing a relationship between the peak
wavelength of laser light in the wavelength band of 2.8 .mu.m (2.7
.mu.m to 3.0 .mu.m) emitted from an optical fiber laser and the
power of the pumping light based upon a comparative example;
[0048] FIG. 7 is a diagram showing energy levels of erbium ions of
an erbium-doped fiber;
[0049] FIG. 8 is a graph showing a relationship between the peak
wavelength of laser light emitted from a conventional optical fiber
laser and the power of the pumping light;
[0050] FIG. 9 is a diagram showing energy levels of erbium ions of
an erbium-doped fiber; and
[0051] FIG. 10 shows Stark levels of the upper level and the lower
level.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0052] In the following, preferred embodiments of the present
invention will be described with reference to the drawings. The
present invention should not be limited to the embodiments
described below, and components in the embodiments may be
appropriately combined with each other.
[0053] FIG. 1 is a schematic view of an exemplary optical fiber
laser according to an aspect of the present invention.
[0054] An optical fiber laser 101 includes a pumping light source
102 and an erbium-doped fiber (also referred to as EDF,
hereinafter) 103 serving as a gain medium.
[0055] The pumping light source 102 may be any one which can emit
pumping light 104 of a wavelength of 980 nm or longer. For example,
it may be a semiconductor laser or titanium-sapphire laser capable
of emitting the pumping light 104 of a wavelength of 980 nm or
longer or a laser module provided with such a laser. As an example,
FIG. 1 shows the pumping light source 102 comprising a
semiconductor laser module 121. Opposed to a laser emission port
121a of the semiconductor laser module 121, a condenser lens 105 is
provided. The condenser lens 105 serves to condense the pumping
light 104 emitted from the semiconductor laser module 121 and to
couple the pumping light 104 to an entrance end face 131 of the EDF
103 with a rear mirror 171, described below, vapor-deposited
thereon.
[0056] The EDF 103 may be any one which is made of a glass
material, such as fluoride glass or silica glass, doped with erbium
(Er), and can serve as a gain medium. The composition of
constituent elements of the EDF 103 is adjusted so that laser light
106 in the wavelength band of 2.8 .mu.m (2.7 .mu.m to 3.0 .mu.m)
can be emitted.
[0057] For example, the EDF 103 may be a double cladded fiber
comprising a core made of a fluoride glass doped with erbium and
having a diameter of 10 .mu.m, a first cladding surrounding the
core and having a rectangular cross section measuring 100 .mu.m by
200 .mu.m, and a second cladding surrounding the first cladding.
The present invention is not limited thereto, and the diameter of
the core, and the shape and size of the claddings can be
arbitrarily determined.
[0058] Here, the "double cladded fiber" refers to an optical fiber
having two layers of cladding around a core. In such a double
cladded fiber, pumping light launched into the fiber propagates in
the first cladding around the core, and laser light produced in the
core propagates in the core.
[0059] The EDF 103 has one end face (also referred to as an
entrance end face) 131 opposed to the condenser lens 105.
[0060] On the entrance end face 131 of the EDF 103, the rear mirror
171 is deposited. On the other end face (also referred to as an
exit end face) 132, an output coupler 172 is provided.
[0061] The rear mirror 171 may be a dielectric multilayer film
filter comprising several to several hundred thin films of
materials different in refractive index, such as SiO.sub.2 or
Ta.sub.2O.sub.5, having a thicknesses of several tens to several
hundred nanometers. The rear mirror 171 is formed on the entrance
end face 131 of the EDF 103 by vapor deposition of the dielectric
multilayer film or the like. The rear mirror 171 allows the pumping
light 104 to pass therethrough and reflects the laser light
produced in the EDF 103.
[0062] The output coupler 172 may be one using Fresnel reflection
at the exit end face 132 as an output coupler or having a
dielectric multilayer film (multilayer film mirror), for example.
The output coupler 172 is formed on the exit end face 132 of the
EDF 103 by vapor deposition of the dielectric multilayer film. The
output coupler 172 reflects some of the laser light 106 emitted
from the EDF 103 and allows the remainder of the laser light 106 to
pass therethrough
[0063] On the side of the exit end face 132 of the EDF 103, a
long-wavelength pass filter 108 is provided. The long-wavelength
pass filter 108 may be one comprising layers of a plurality of
oxides of different refractive index. The long-wavelength pass
filter 108 is provided in the propagation path of the laser light
106 emitted from the output coupler 172. The long-wavelength pass
filter 108 allows only part of the laser light 106 emitted from the
output coupler 172, which has a wavelength of 2.56 .mu.m or longer,
to pass therethrough to be output.
[0064] In the optical fiber laser 101, the rear mirror 171 and the
output coupler 172 serve as reflecting mirrors. In addition, since
the rear mirror 171 and the output coupler 172 are opposed to each
other with the EDF 103 interposed therebetween, the EDF 103, the
rear mirror 171 and the output coupler 172 cooperatively function
as a resonator 107.
[0065] In the optical fiber laser 101, the pumping light source 102
emits the pumping light 104 of a wavelength of 980 nm or longer.
The pumping light 104 is condensed by the condenser lens 105 and
coupled to the entrance end face 131 of the EDF 103 with the rear
mirror 171 vapor-deposited thereon. Then, the EDF 103 is excited by
the incident pumping light 104 and emits the laser light 106 in the
wavelength band of 2.8 .mu.m (2.7 .mu.m to 3.0 .mu.m) from the exit
end face 132.
[0066] In the optical fiber laser 101, the laser light 106 in the
wavelength band of 2.8 .mu.m (27 .mu.m to 3.0 .mu.m) is emitted
from the EDF 103 by varying the wavelength of the pumping light 104
in the range of 966 nm to 1011 nm. The result of measurement of the
power of the laser light 106 will be described below.
[0067] FIG. 2 shows a relationship between the power of the laser
light 106 in the wavelength band of 2.8 .mu.m (2.7 .mu.m to 3.0
.mu.m) emitted from the EDF 103 and the wavelength of the pumping
light 104.
[0068] As the EDF 103, a single mode fiber having a double cladded
structure and made of Zr--Ba--La--Al--Na fluoride (that is a
fluoride glass, also referred to as ZBLAN) doped with erbium is
used. The core diameter of the EDF 103 is 10 .mu.m, and the first
cladding has a rectangular cross section measuring 100 .mu.m by 200
.mu.m. In addition, the power of the pumping light 104 incident on
the EDF 103 is constant; specifically, 2 W.
[0069] When the wavelength of the pumping light 104 is between 965
nm and 985 nm, the longer the wavelength of the pumping light 104,
the higher the power of the produced laser light 106. The power of
the laser light 106 reaches a maximum when the pumping light 104
has a wavelength of 985 nm.
[0070] As can be seen from the drawing, when the wavelength of the
pumping light 104 is between 985 nm and 1000 nm, the power of the
laser light 106 is equal to or higher than 240 mW, and the laser
light 106 produced has higher intensities than the laser light
caused by the pumping light 104 of a wavelength of 980 nm.
[0071] The optical fiber laser according to this aspect of the
present invention can emit the laser light 106 in the wavelength
band of 2.8 .mu.m (2.7 .mu.m to 3.0 .mu.m) with a high emission
efficiency if the wavelength of the pumping light 104 is set at 980
nm or longer. The wavelength of the pumping light 104 preferably
falls within the range of 985 nm to 1000 nm, and, in that case, the
laser light 106 can be produced which has a higher emission
efficiency and higher power than those of the laser light produced
using conventional pumping light of a wavelength of shorter than
980 nm.
[0072] In addition, the EDF 103 made of a fluoride glass
substantially does not absorb light in the wavelength band of 2.8
.mu.m (2.7 .mu.m to 3.0 .mu.m). Thus, the propagation loss of the
laser light 106 in the wavelength band of 2.8 .mu.m (2.7 .mu.m to
3.0 .mu.m) can be low. Therefore, the laser light 106 in the
wavelength band of 2.8 .mu.m (2.7 .mu.m to 3.0 .mu.m) of a much
higher power can be emitted with a much higher efficiency.
[0073] In the case in which the EDF 103 has a double cladded
structure, the first cladding, which allows the pumping light 104
pass therethrough, serves as an excitation region. Thus, a wide
area of the entrance end face 131 of the EDF 103 including the
first cladding can be irradiated with the pumping light 104, and a
semiconductor laser of a high power can be used as the pumping
light source 102, for example. Thus, the laser light 106 in the
wavelength band of 2.8 .mu.m (2.7 .mu.m to 3.0 .mu.m) of a high
power can be emitted relatively easily.
[0074] Here, the technical scope of the present invention is not
limited to the embodiment described above, and many alterations are
possible without departing from the spirit of the present
invention.
[0075] For example, the pumping light source 102 may be provided
with a lead fiber at the light emission port, the lead fiber being
for the pumping light to exit and connected to the EDF 103 by butt
joining.
[0076] The core radius, reeve index distribution, erbium density
distribution and the like of the EDF 103 are not particularly
limited. For example, the EDF 103 may be a multimode fiber or one
having a first cladding which is a square or circle in cross
section.
[0077] In addition, a Q switch or the like may be provided on the
side of the entrance end face 131 or exit end face 132 of the EDF
103 to enable pulse oscillation of the laser light 106.
[0078] FIG. 3 is a schematic diagram showing an exemplary optical
fiber laser according to another aspect of the present
invention.
[0079] In general, an optical fiber laser 200 includes a first
pumping light source 201, a second pumping light source 202, an
erbium-doped glass fiber (also referred to as an EDF, hereinafter)
203 serving as a gain medium, collimator lenses 204 and 205, a
multilayer film filter 206, a condenser lens 207, a rear mirror
208, an output coupler 209, and a long-wavelength pass filter
210.
[0080] The first pumping light source 201 may be any one which can
emit light (first pumping light 221) in a wavelength band which
allows occurrence of ground state absorption in the EDF 203 For
example, the first pumping light source 201 may be a semiconductor
laser or titanium-sapphire laser capable of emitting the first
pumping light 221 in a wavelength band of 980 nm that is a
wavelength range of 960 nm to 1020 nm, or a laser module provided
with such a laser.
[0081] Emitting the first pumping light 221 into the EDF 203 can
cause ground state absorption in the EDF 203. Thus, erbium ions in
the ground state (.sup.41.sub.15/2) can be excited to the upper
level (.sup.4I.sub.11/2), thereby emitting laser light 223 having a
wavelength range of 2.8 .mu.m to 2.9 .mu.m.
[0082] The second pumping light source 202 may be any one which can
emit light (second pumping light 222) in a wavelength band which
allows occurrence of excited state absorption from the lower level
(.sup.4I.sub.13/2) in the EDF 203. For example, the second pumping
light source 202 may be a semiconductor laser or titanium-sapphire
laser capable of emitting the second pumping light 222 in a
wavelength band of 790 nm that is a wavelength range of 780 nm to
792 nm, or a laser module provided with such a laser.
[0083] Emitting the second pumping light 222 into the EDF 203 can
cause excited state absorption from the lower level
(.sup.4I.sub.13/2) in the EDF 203. However, no excited state
absorption occurs from the upper level (.sup.4I.sub.11/2).
[0084] The EDF 203 may be any one which is made of a glass
material, such as fluoride glass or silica glass, doped with erbium
(Er) and can serve as a gain medium. The composition of constituent
elements of the EDF 203 is adjusted so that laser light 223 in the
wavelength band of 2.8 .mu.M (2.7 .mu.m to 3.0 .mu.m) can be
emitted.
[0085] For example, the EDF 203 may be a double cladded fiber
comprising a core made of a fluoride glass doped with erbium and
having a diameter of 10 .mu.m, a first cladding surrounding the
core and having a rectangular cross section measuring 100 .mu.m by
200 .mu.m, and a second cladding surrounding the first cladding.
The present invention is not limited thereto, and the diameter of
the core, and the shape and size of the claddings can be freely
determined.
[0086] Here, the "double cladded fiber" refers to an optical fiber
having two layers of cladding around a core In such a double
cladded fiber, pumping light launched into the fiber propagates in
the first cladding around the core, and laser light produced
therein propagates in the core.
[0087] Tee collimator lens 204 may be a plano-convex lens made of
an optical glass or the like. The collimator lens 204 is disposed
to have the planar face opposed to the first pumping light source
201 and the convex face opposed to the multilayer film filter 206.
This arrangement allows the pumping light emitted from the first
pumping light source 201 to be collimated and the light reflected
on the multilayer film filter 206 to be condensed.
[0088] The collimator lens 205 may be a plano-convex lens made of
an optical glass or the like. The collimator lens 205 is disposed
to have the planar face opposed to the second pumping light source
202 and the convex face opposed to the multilayer film filter 206.
This arrangement allows the pumping light emitted from the second
pumping light source 202 to be collimated and the light reflected
on the multilayer film filter 206 to be condensed.
[0089] The multilayer film filter 206 may be a dielectric
multilayer film filter comprising several to several hundred thin
films of materials different in refractive index, such as SiO.sub.2
or Ta.sub.2O.sub.5, having thicknesses of several tens to several
hundred nanometers. The multilayer film filter 206 is disposed in
the propagation paths of the first pumping light 221 having passed
through the collimator lens 204 and the second pumping light 222
having passed through the collimator lens 205 and, thus, can
multiplex the first pumping light 221 and the second pumping light
222.
[0090] The condenser lens 207 may be a plano-convex lens made of an
optical glass or the like. The condenser lens 207 is disposed in
the propagation path of the first pumping light 221 and second
pumping light 222 multiplexed by the multilayer film filter 206 in
such a manner that the convex face is opposed to the multilayer
film filter 206 and the planar face thereof is opposed to an
entrance end face 203a of the EDF 203. This arrangement allows the
first pumping light 221 and second pumping light 222 multiplexed by
the multilayer film filter 206 to be condensed onto the rear mirror
208 provided on the entrance end face 203a of the EDF 203.
[0091] The rear mirror 208 may be a dielectric multilayer film
filter comprising several to several hundred thin films of
materials of different refractive index, such as SiO.sub.2 or
Ta.sub.2O.sub.5, having thicknesses of several tens to several
hundred nanometers. The rear mirror 208 is formed on the entrance
end face 203a of the EDF 203 by vapor deposition of the dielectric
multilayer film or the like. The rear mirror 208 allows the first
pumping light 221 and second pumping light 222 to pass therethrough
and reflects the laser light produced in the EDF 203. Furthermore,
the rear mirror 208 used preferably has a transmittance of 95% or
higher for the first pumping light 221 and second pumping light 222
and a reflectance of 99% or higher for the laser light.
[0092] The output coupler 209 may be one using Fresnel reflection
at the end face as an output coupler or having a dielectric
multilayer film, for example. The output coupler 209 is formed on
an exit end face 203b of the EDF 203 by vapor deposition of the
dielectric multilayer film. The output coupler 209 reflects some of
the laser light 223 emitted from the EDF 203 and allows the
remainder of the laser light 223 to pass therethrough. The output
coupler 209 used preferably has a reflectance of 3% to 90% for the
laser light.
[0093] The long-wavelength pass filter 210 may be one comprising
layers of a plurality of oxides of different refractive index. The
long-wavelength pass filter 210 is provided in the propagation path
of the laser light 223 emitted from the output coupler 209. The
long-wavelength pass filter 210 allows only part of the laser light
223 emitted from the output coupler 209 which has a wavelength of
2.56 .mu.m or longer to pass therethrough.
[0094] In the optical fiber laser 200, the rear mirror 208 and the
output coupler 209 serve as reflecting mirrors. In addition, since
the rear mirror 208 and the output coupler 209 are opposed to each
other with the EDF 203 interposed therebetween, the EDF 203, the
rear mirror 208 and the output coupler 209 cooperatively function
as a resonator 215.
[0095] In the optical fiber laser 200, the first pumping light
source 201 emits the first pumping light 221 in a wavelength band
of 980 nm (960 nm to 1020 nm), and, simultaneously, the second
pumping light source 202 emits the second pumping light 222 in a
wavelength band of 790 nm (780 nm to 792 nm). The first pumping
light 221 and second pumping light 222 are multiplexed by the
multilayer film filter 206, and the resulting multiplexed light is
condensed by the condenser lens 207 and coupled onto the entrance
end face 203a of the EDF 203 with the rear mirror 208.
[0096] Then, the EDF 203 is excited by the incident first pumping
light 221 and second pumping light 222 and emits the laser light
223 in the wavelength band of 2.8 .mu.m (2.7 .mu.m to 3.0 .mu.m)
from the exit end face 203b provided with the output coupler
209.
[0097] Now, the first pumping light 221 and second pumping light
222, which are essential in the optical fiber laser according to
the latter aspect of the present invention, will be described in
detail based on the light emission principles in the optical fiber
laser 200.
[0098] When the first pumping light 221 in a wavelength band of 980
nm (960 nm to 1020 nm) emitted from the first pumping light source
201 is launched into the EDF 203, ground state absorption (also
refereed to as GSA, hereinafter) occurs, and erbium ions in the
ground state (.sup.4I.sub.15/2) are excited to the upper level
(.sup.4I.sub.11/2). Then, when the erbium ions transit from the
upper level (.sup.4I.sub.11/2) to the lower level
(.sup.4I.sub.13/2), the laser light 223 in a wavelength range of
2.8 .mu.m to 2.9 .mu.m is emitted.
[0099] The first pumping light 221 in a wavelength band of 980 nm
(960 nm to 1020 nm) has a wavelength that allows excitation of the
EDF 203 to cause emission of the laser light 223 in a wavelength
range of 2.8 .mu.m to 2.9 .mu.m. Among others, the light preferably
is in a wavelength range of 960 nm to 1020 nm, which allows
emission of the laser light 223 in a wavelength range of 2.8 .mu.m
to 2.9 .mu.m of high power with a high efficiency.
[0100] FIG. 10 shows the upper level (.sup.4I.sub.11/2) and the
lower level (.sup.4I.sub.13/2) of the energy levels of erbium ions
in the EDF. As is known, the upper level (.sup.4I.sub.11/2) and the
lower level (.sup.4I.sub.13/2) are each composed of a plurality of
Stark levels shortly spaced from each other.
[0101] If, as with the conventional cases, only a pumping light in
the wavelength band of 980 nm (960 nm to 1020 nm) is used as
pumping light, when the intensity (power) of the pumping light is
increased, the number of excited erbium ions increases, and the
number of erbium ions occupying the upper level (.sup.4I.sub.11/2)
and the lower level (.sup.4I.sub.13/2) of the excited levels also
increase. Since lower Stark levels are occupied by erbium ions
earlier than higher ones in each energy level, the lower Stark
levels are occupied by erbium ions.
[0102] Thus, when erbium ions transit from the upper level
(.sup.4I.sub.11/2) to the lower level (.sup.4I.sub.13/2), the
erbium ions transit to the higher Stark levels of the lower level
(.sup.4I.sub.13/2), and, accordingly, the higher Stark levels
contribute to light emission Thus, the interval between the upper
level (.sup.4I.sub.11/2) to which the erbium ions transit and the
lower level (.sup.4I.sub.13/2) is reduced, and the wavelength of
the emitted light is shifted toward longer wavelengths.
[0103] Besides the excitation from the ground state
(.sup.4I.sub.15/2) due to GSA, excitations of erbium ions include
excitations from the upper level (.sup.4I.sub.11/2) or lower level
(.sup.4I.sub.13/2) to still higher levels due to excited state
absorption (also referred to as ESA, hereinafter).
[0104] The optical fiber laser according to the latter aspect of
the present invention uses the second pumping light 222 in a
wavelength band of 790 nm (780 nm to 792 nm) in addition to the
first pumping light 221 in a wavelength band of 980 nm (960 nm to
1020 nm) and, thus, causes ESA in the erbium ions at the lower
level (.sup.4I.sub.13/2) to excite the erbium ions to another
energy level. Thus, the number of erbium ions occupying the lower
level (.sup.4I.sub.13/2) is reduced, enabling the lower Stark
levels of the lower level (.sup.4I.sub.13/2) to contribute to
emission of the laser light
[0105] In this way, even if the power of the pumping light is high,
the laser light 223 of a constant wavelength can be stably emitted
without any shift of the peak wavelength thereof.
[0106] FIG. 4 is a graph showing absorption spectra of the GSA by
the erbium ions in the ground state (.sup.4I.sub.15/2), the ESA by
the erbium ions at the upper level (.sup.4I.sub.11/2) and the ESA
by the erbium ions at the lower level (.sup.4I.sub.13/2). FIG. 4,
which shows plots of absorptions with respect to the wavelength of
the pumping light, is cited from Applied Physics B, 1998, Vol. 67,
pp. 23-28.
[0107] As can be seen from FIG. 4, when the wavelength of the
pumping light is between 780 nm and 812 nm, ESA occurs in the lower
level (.sup.4I.sub.13/2). In addition, it can be seen that when the
wavelength of the pumping light is between 780 nm and 792 nm, ESA
occurs in the lower level (.sup.4I.sub.13/2) but doesn't occur in
the upper level (.sup.4I.sub.11/2).
[0108] As the second pumping light 222 in a wavelength band of 790
nm (780 nm to 792 nm), the optical fiber laser according to the
latter aspect of the present invention uses light having a
wavelength that allows occurrence of ESA by the erbium ions at the
lower level (.sup.4I.sub.13/2) (that is, in a wavelength range of
780 nm to 812 nm in FIG. 4). Among others, preferably used is light
in a wavelength range of 780 nm to 792 nm, which can cause ESA by
the erbium ions at the lower level (.sup.4I.sub.13/2) without
causing ESA by the erbium ions at the upper level
(.sup.4I.sub.11/2).
[0109] Thus, the number of the erbium ions occupying the lower
level (.sup.4I.sub.13/2) can be reduced, so that the lower Stark
levels of the lower level (.sup.4I.sub.13/2) can be made to
contribute to light emission. Furthermore, since the second pumping
light does not cause ESA to occur in the erbium ions at the upper
level (.sup.4I.sub.11/2) which contribute to emission of the laser
light in a wavelength range of 2.8 .mu.m to 2.9 .mu.m, the number
of the erbium ions at the upper level (.sup.4I.sub.11/2) does not
decrease.
[0110] In this way, by concurrently using the fist pumping light
221 in a wavelength band of 980 nm (960 nm to 1020 nm) and the
second pumping light 222 in a wavelength band of 790 nm (780 nm to
792 nm), the laser light 223 of a constant wavelength can be stably
emitted from the EDF 203 with high efficiency while preventing the
peak wavelength thereof from being shifted toward longer
wavelengths, even if the sum of the intensities of the first
pumping light 221 and second pumping light 222 is high.
[0111] Here, the technical scope of the present invention is not
limited to the embodiments described above, and many alterations
are possible without departing from the spirit of the present
invention.
[0112] For example, the first pumping light source 201 and second
pumping light source 202 may be provided with a lead fiber at the
light emission port thereof so that the first pumping light 221 in
a wavelength band of 980 nm (960 nm to 1020 nm) and second pumping
light 222 in a wavelength band of 790 nm (780 nm to 792 nm) are
emitted via the respective lead fibers. In this case, the lead
fibers can be connected to a two-input one-output WDM coupler to
multiplex the first pumping light 221 in a wavelength band of 980
nm (960 nm to 1020 nm) and second pumping light 222 in a wavelength
band of 790 nm (780 nm to 792 nm) so that they can be emitted
through one optical fiber.
[0113] Furthermore, one pumping light source that emits two light
beams of different wavelengths may be used.
[0114] Furthermore, the core radius, refractive index distribution,
erbium density distribution and the like of the EDF 203 are not
particularly limited. For example, the EDF 203 may be a multimode
fiber or one having a first cladding which is a square or circle in
cross section.
[0115] In the following, the optical fiber laser according to the
latter aspect of the present invention will be described in more
detail. However, the present invention is not limited to the
following example
EXAMPLE
[0116] The optical fiber laser 200 shown in FIG. 3 was
prepared.
[0117] The EDF 203 used was a single mode fiber made of a fluoride
glass doped with erbium, having a core of a diameter of 10 .mu.m
and having a double cladded structure whose first cladding has a
rectangular cross section measuring 100 .mu.m by 200 .mu.m.
[0118] The wavelength of the first pumping light 221 launched into
the EDF 203 was 980 nm, and the wavelength of the second pumping
light 222 was 790 nm.
[0119] The relationship between the peak wavelength of the laser
light 223 in the wavelength band of 2.8 .mu.m (2.7 .mu.m to 3.0
.mu.m) emitted from the optical fiber laser 200 and the sum of the
intensities of the first pumping light 221 and second pumping light
222 was examined. The results are shown in FIG. 5.
[0120] As can be seen from FIG. 5, even if the sum of the
intensities of the first pumping light 221 and second pumping light
222 is high, specifically, 1.5 W or higher, the peak wavelength of
the emitted laser light 223 is substantially kept constant around
2760 mm. Thus, is was confirmed that, even if the sum of the
intensities of the first pumping light 221 and second pumping light
222 varies, the laser light 223 of a constant wavelength can be
stably emitted while preventing the peak wavelength thereof from
being shifted toward longer wavelengths.
COMPARATIVE EXAMPLE
[0121] The optical fiber laser in the Comparative Example differs
from the optical fiber laser 200 in the above Example in that the
second pumping light source 202 emits the second pumping light 222
of a wavelength of 532 nm.
[0122] As in the above Example, the relationship between the peak
wavelength of the laser light 223 in the wavelength band of 2.8
.mu.m (2.7 .mu.m to 3.0 .mu.m) emitted from the optical fiber laser
and the sum of the intensities of the first pumping light 221 and
second pumping light 222 was examined. The results are shown in
FIG. 6.
[0123] From the result shown in FIG. 6, it was confirmed that, when
the sum of the intensities of the first pumping light 221 and
second pumping light 222 is high, specifically, 1.5 W or higher,
the peak wavelength of the emitted laser light 223 is shifted
toward longer wavelengths.
[0124] This is due to the fact that the pumping light of a
wavelength of 532 nm causes GSA but does not cause ESA. Thus, in
the optical fiber laser of two-wavelength excitation type which
uses a pumping light having a wavelength which can cause only GSA,
the wavelength of the emitted laser light 223 is shifted toward
longer wavelengths. Accordingly, it can be seen that, even if two
kinds of pumping light which can cause only GSA are launched into
the EDF 203, the wavelength shift of the emitted laser light 223
cannot be suppressed
[0125] It is contemplated that numerous modifications may be made
to the embodiments and implementations of the present inventions
without departing from the spirit and scope of the invention as
defined in the following claims.
* * * * *